In the manufacture of semiconductor devices and the like, a stepping or step-and-scan projection exposure device (aligner) has been used in which a pattern of a reticle (photomask) is transferred onto each shot region on a wafer provided with a photoresist through a projection optical system.
The theoretical limit of the resolution of the projection optical system provided in the projection exposure device increases as the exposure wavelength used becomes shorter and the numerical aperture of the projection optical system becomes greater. Therefore, the exposure wavelength which is the wavelength of radiation used in the projection exposure device has been reduced along with scaling down of integrated circuits, and the numerical aperture of the projection optical system has been increased.
The depth of focus is also important for exposure in addition to the resolution. The theoretical limits of the resolution R and the depth of focus δ are respectively shown by the following expressions.R=k1·λ/NA  (i)δ=k2·λ/NA2  (ii)
In the above expressions, λ is the exposure wavelength, k1 and k2 are process coefficients, and NA is the numerical aperture of the projection optical system, which is defined by the following expression (ii′) when the refractive index of air is 1. Specifically, when obtaining the same resolution R, a larger depth of focus δ is obtained by using radiation with a shorter wavelength.NA=sin θ(θ=maximum incident angle of exposure light with respect to resist surface)  (ii′)
As described above, the demand for scaling down of integrated circuits has been satisfied by reducing the wavelength of the exposure light source and increasing the numerical aperture. At present, mass production of a 1L1S (1:1 line and space) 90-nm half-pitch node is studied using an ArF excimer laser (wavelength: 193 nm) as the exposure light source. However, it is difficult to achieve the next generation 65-nm half-pitch node or 45-nm half-pitch node using only the ArF excimer laser. Therefore, use of a light source with a shorter wavelength such as an F2 excimer laser (wavelength: 157 nm) or an extreme ultraviolet (EUV) laser (wavelength: 13 nm) has been studied for the next generation technology. Note that it is difficult to use these light sources under the present situation due to technological difficulty.
In the above exposure technology, a photoresist film is formed on the surface of the exposure target wafer, and the pattern is transferred to the photoresist film. In a related-art projection exposure device, the space in which the wafer is placed is filled with air having a refractive index of 1, or nitrogen. When the space between the wafer and the lens of the projection exposure device is filled with a medium having a refractive index of n, the theoretical limits of the resolution R and the depth of focus δ are shown by the following expressions.R=k1·(λ/n)/NA  (iii)δ=k2·nλ/NA2  (iv)
In the above expressions, NA is not the actual numerical aperture of the projection optical system, but refers to the constant defined by the above expression (ii′) (the numerical aperture NA′ of the projection optical system is expressed by NA′=n sin θ (n is the same as defined above)).
The above expressions mean that the limit of the resolution and the depth of focus can be theoretically increased by 1/n and n, respectively, by filling the space between the lens of the projection exposure device and the wafer with a liquid having a refractive index of n to provide an appropriate optical system. For example, when using water as the above medium in the ArF process, since water has a refractive index n for light with a wavelength of 193 nm of 1.44, an optical system can be theoretically designed in which the resolution R is 69.4% (R=k1·(λ/1.44)/NA) and the depth of focus is 144% (δ=k2·1.44λ/NA2) of the values during exposure using air or nitrogen as the medium.
The above projection exposure method in which the effective wavelength of exposure radiation is reduced to transfer a more minute pattern is called an immersion exposure method. The immersion exposure method is considered to be an essential technology for lithography with reduced dimensions, particularly for lithography with dimensions of several ten nanometers (Japanese Patent Application Laid-open No. 11-176727).
As the liquid provided between the lens of the projection optical system and the substrate in the immersion exposure method, use of pure water has been studied for the ArF excimer laser, and use of a fluorine-based inert liquid has been studied for the F2 excimer laser due to high transparency to light with a wavelength of 157 nm.
Pure water is easily available in a semiconductor manufacturing factory and does not pose an environmental problem. Moreover, since the temperature of pure water can be easily adjusted, thermal expansion of the substrate due to heat generated during exposure can be prevented. Therefore, pure water has been used as the ArF immersion liquid (International Publication No. WO 99/49504), and is certain to be used for mass production of 65-nm half-pitch node devices.
A liquid has also been known in which methyl alcohol or the like is added to pure water as an additive which decreases the surface tension and increases the surface activity of pure water (Japanese Patent Application Laid-open No. 10-303114).
On the other hand, pure water may permeate the photoresist film, whereby shape deterioration may occur in which the photoresist pattern has a T-top cross-sectional shape, or the resolution may be decreased. Moreover, water-soluble components such as a photoacid generator and basic additive of the photoresist and acids generated upon exposure may be eluted (dissolved) in water, thereby causing shape deterioration such as a T-top shape, a decrease in resolution and depth of focus, bridge defects, defects in the developed pattern, or contamination of the lens surface. The elution of these components into the liquid also causes contamination of the liquid, thereby making it difficult to recycle the liquid. Therefore, complicated purification must be frequently carried out.
An upper layer film may be formed on the photoresist film in order to protect the photoresist film from water. However, the upper layer film may exhibit insufficient transparency for exposure light or insufficient intermixing properties with the photoresist film, or may complicate the manufacturing process. It has been reported that CaF2 used as the lens material is eroded by water (non-patent document: Nikkei Microdevice, April 2004, p. 77). This makes it necessary to coat the lens surface with a coating material.
On the other hand, since the resolution obtained using pure water is limited to the value about 1.44 times that of ArF dry exposure, as indicated by the above expression (iii), it may become difficult to use pure water for the next generation technology particularly with a half pitch of 45 nm or less.
As described above, a liquid having a refractive index for light with an exposure wavelength (e.g. 193 nm) greater than that of pure water and exhibiting high transparency to light with such a wavelength has been demanded for the next generation immersion exposure method. This liquid must not adversely affect the photoresist film due to elution of the additive from the photoresist film, dissolution of the resist film, deterioration of the pattern, and the like, and must not erode the lens. Moreover, since the use of polarized light as the exposure light has been studied along with an increase in NA accompanying introduction of immersion exposure, the liquid is also required to not rotate the polarization direction due to optical activity or the like.
In order to achieve the above objects, an attempt has been made to increase the refractive index by dissolving various salts in water (non-patent document: Proc. SPIE Vol. 5377 (2004) p. 273), for example. However, this approach has problems such as difficulty in controlling the salt concentration or occurrence of development defects or lens contamination due to elution of water-soluble components.
A fluorine-based inert liquid such as perfluoropolyether which has been studied for F2 exposure has a low refractive index for light with a wavelength of 193 nm, for example. Therefore, it is difficult to use such a fluorine-based inert liquid at such a wavelength. An organic bromide or iodide known as a microscope immersion exposure liquid due to the high refractive index for light with a wavelength of 589 nm exhibits poor transparency to light with a wavelength of 193 nm and insufficient stability for the photoresist film, for example.